Step 1Combine the sugar, flour, nutmeg and salt in a bowl. Stir in the vanilla, then work in the butter with a pastry blender or fork until crumbly. Set aside.
Step 1Heat the oven to 350 degrees. Butter a 9-inch tube pan.
Step 2Stir together the flour, baking powder, baking soda, nutmeg and salt in a bowl. Set aside.
Step 3Beat together the butter and sugar in a mixing bowl until light and fluffy. Beat in the eggs, one at a time. Beat in the vanilla.
Step 4Beat in half the flour mixture until creamy, then beat in the sour cream until blended. Beat in the remaining flour mixture until combined. Stir in the rhubarb.
Step 5Spoon the batter into the tube pan. Top with the crumble mixture. Bake until a toothpick inserted in the center comes out clean, about 1 hour. Cool the cake in the pan slightly, then remove to a wire rack to cool completely.